1. Field of the Invention
The present invention relates to fuel delivery systems and, more specifically, systems for delivering liquified natural gas from a fuel tank to an engine.
2. Present State of the Art
The increasing output of automobile emissions and the decreasing supply of oil reserves has motivated the search for alternative motor vehicle fuels. One alternative fuel is natural gas. Natural gas is clean burning and can be stored in a dense, high energy liquid form. Liquefying natural gas is accomplished by cooling the natural gas to a cryogenic temperature, typically below -260.degree. F., which condenses the gas into a liquid. Working with and keeping natural gas at a cryogenic temperature, however, creates inherent problems. Furthermore, natural gas, prior to combustion, is a harmful greenhouse gas. As such, it is important that the escape of any natural gas be minimized to prevent increased damage to the atmosphere.
In one approach to using natural gas in automobiles, the natural gas is initially stored in large tanks at refueling stations. The large tanks maintain the fuel at a cryogenic temperature so as to keep the natural gas in a dense liquid state. Smaller insulated fuel tanks are located within the automobiles and can be filled with the liquified natural gas at a refueling station. As discussed above, it is desirable to store the naturel gas in a liquified state. It is also beneficial, however, to have the automobile fuel tank sufficiently pressurized so that the fuel therein will automatically flow to the vehicle engine. Although a pump can be used to deliver the fuel to the engine, use of a pump requires energy. Furthermore, pumping natural gas at cryogenic temperatures has been found problematic.
In one approach to obtaining the desired pressure within the automobile fuel tank, systems have been incorporated into refueling stations which warm the liquified natural gas as it is pumped into the automobile fuel tank. By heating the liquified natural gas to a desired temperature, a portion of the liquified natural gas vaporizes within the fuel tank to produce the desired pressure. The pressure created within the fuel tank as a result of warming the fuel is call "saturation pressure". Although this process achieves the desired objective, it also produces several problems.
For example, the systems for heating the natural gas at the refueling station are time consuming and expensive to operate and build. Furthermore, as a result of warming the natural gas, less natural gas can be stored within the fuel tank. In addition, since all of the natural gas that is pumped into the automobile fuel tank is heated, the fuel must be used relatively quickly to prevent having to vent any of the natural gas to the atmosphere. That is, although the automobile fuel tank is insulated, once the liquified natural gas is pumped therein, the fuel begins to slowly warm towards an equilibrium with the outside temperature. As the fuel warms, the pressure within the tank increases. Once the tank reaches a control pressure, a pressure relief valve is opened allowing a portion of the natural gas to escape into the atmosphere, thereby decreasing the internal pressure. The time period that a tank can hold natural gas without having to vent is called the "hold time." As previously discussed, releasing natural gas into the atmosphere is both wasteful and potentially harmful.
In contrast, if the natural gas is consumed too quickly, the pressure within the fuel tank can drop below the required operating pressure. As liquified natural gas is consumed, the volume of the vapor holding portion of the fuel tank is increased. As this volume increases, a portion of the liquified natural gas is vaporized to fill the space within the fuel tank. Vaporization of natural gas is an endothermic process which absorbs heat. Accordingly, as the natural gas within the fuel tank is vaporized, the temperature and thus pressure within the fuel tank decreases. If liquified natural gas is consumed too quickly, the pressure will drop below the operating pressure.
In an alternative approach to pressurizing the automobile fuel tank, a heater is directly coupled with the automobile fuel tank for heating the liquified natural gas therein. The problem with this approach is that it takes both time and energy to heat the fuel within the fuel tank. Furthermore, the same problem exists of having to use the natural gas relatively quickly to prevent having to vent portions of the natural gas to the atmosphere.
Other problems in conventional liquified natural gas systems relate to the lines extending from the fuel tank to the engine. Many of the prior art systems require the use of electronic switches, solenoids, and computers to operate them. The use of such electronics is expensive, increases the complexity of the system, decreases the reliability of the system, and consumes large amounts of energy.
The same problems as discussed above for vehicles are also applicable to using natural gas to run engines that are not vehicle related.